To generate a favorable climate in a room large area illumination devices are needed. Nowadays often gas discharge lamps are used to generate a homogeneous light, illuminating large parts of the room. Unfortunately, discharge lamps are known to be costly and quite inefficient. To overcome this disadvantage, the use of organic light emitting diodes (OLED) is appropriate. The advantage of the OLED is that it is a homogeneous light source with potentially low costs and high efficiency. Organic light emitting devices (materials and structures) are known in the art, for instant as disclosed in WO2005/053053 A1, the disclosure of which is incorporated herein by reference for all purposes.
However, large size OLEDs suffers the problem of brightness inhomogeneity. The brightness inhomogeneity is due to the fact that the OLEDs comprise thin film electrodes, with a thickness of around a few hundred nm. Therefore, the thin film electrodes have a relative high sheet resistance. As OLEDs are driven at low voltage and high currents, the high sheet resistance results in a voltage drop along the electrodes, causing the brightness inhomogeneity. This disadvantage is getting worse with increasing OLED size.